Variable frequency oscillatory system



Aug. 17, 1948. L. F. CURTIS VARIABLE FREQUENCY OS CILLATORY SYSTEI 2Shoots-Shut 1 Filed Nov. 27, 1945 A ormev.

Aug. 17, 1948. u 1' 5 2,447,316

VARIABLE FREQUENCY OSCILLATORY SYSTEM Filed Nov. 27, 1945 2 Shoots-Shoot2 Ralutlve Distance Relative Dlstunco F'"? 19. ll

INVENTOR.

LES IE F. CURTIS,

A TORNEY.

Patented Aug. 17, 1948' UNITED STATES PATENT OFFICE v VARIABLEOSCILLATORY Leslie new, Great Neck, N. Y., assignor, by mesneassignments, to Hazeitine Research, Inc., Chicago, 111., a corporationof Illinois Application November 27, 1945, Serial No. 631,053

. v 9 Claims. 1 This invention relates to variable-frequency oscillatorysystems 01! the type in which variations oi. a coupling effect betweeninput and output terminals 01 an impedance network of the system areutilized to produce a change of the oscillation frequency. While theinvention is of. general utility, it is particularly suitable for use inan indicating system to indicate the presence of a hidden mass ofmaterial of such nature as to have a different effect on the impedancenetwork of the oscillatory system than does the substance oi! itssurroundings, and will be described in that connection.

The present application is a continuation oi abandoned applicationSerial No. 425,155, flied December 31, 1941, entitled Electricalindicating system, which is assigned to the same assignee as the presentapplication.

It is frequently desirable to detect the presence of a hidden mass ofmaterial having a characteristic diiierent from that of itssurroundings. For

example, it may be desirable to detect, by anelectrical arrangement, thepresence of a hidden mass of material having electrical permeabilitydifferent from its surroundings. Also, it may be desirable to detect, byan arrangement of the same type, the presence of a hidden mass ofmaterial having high electrical conductivity which causes a somewhatsimilar reaction in an electrical circuit. To give a concrete example,it may be desired to locate a metal object such as a pipe or conductorwhich is buried in the;

ground. Various arrangements have been proposed to detect the presenceof such buried objects. Thus, it has been proposed to utilize the changein the reactive efiect of an exploring inductor, due to the presence ofa hidden con-- ductive mass of material, upon a reactance networkincluding the exploring inductor, to determine the presence of such massof material. However, in arrangements this sort the change of thereactive efiect of the exploring inductor on I the network, due to thepresence of a hidden mass of material of the type under consideration,varies approximately inversely as the sixth power of the distancebetween the exploring inductor and the hidden mass of material. Priorart arrangements of the type mentioned above therefore have not beennearly as sensitive as is desired. It is, therefore, desirable toprovide an improved variable-frequency oscillatory system havingexceptionally high sensitivity to variations of a coupling effectbetween input and output terminals or an impedance network includedinthe system and one which is particularly suitable for use in a systemfor detecting the presence of a oi material oi such natureas to have a 2diil'erent eflect on the impedance network than does the substancesurrounding the material.

It isan object of the present invention to provide a new, improved andvery sensitive variablefrequency oscillatory system the frequency ofoscillation of which changes rapidly with impedance variations of animpedancemeans included in an impedance network oi the system.

It is a further object 01! the present invention to provide an improvedvariable-frequency oseillatory system particularly suitable for use inan indicating system for detecting the presence 01' a hidden mass ofmaterial of such nature as to have a different eflfect on an impedancenetwork than does the substance surrounding the material.

, It is still another objection of the invention to provide a new andimproved variable-frequency oscillatory system which includes anexploring inductor so disposed that a variation of its reactive effectupon a network including the inductor is utilized to effect asubstantial change of the irequency of oscillation of the system.

In accordance with a particular form oi." the invention, avariable-frequency oscillatory system comprises an impedance networkhaving input .and output terminals and including a passivemutual-reactance network normally balanced to provide a substantiallyzero value of teed-back voltage from the input to the output terminals.

The mutual-reactance network is responsive to a frequency-controlcondition extemal thereto in space for modifying the balance of suchnetwork to provide feed-back voltage from the input to the outputterminals. The system also includes an oscillator for generatingoscillations, means for applying the oscillations to the aforesaid inputterminals, means for amplifying the oscillations translated through thenetwork to the aforesaid output terminals, and means for applying theamplified oscillations back to the oscillator in such phase as to shiftthe oscillator frequency in response to the aforesaid modification ofthe network balance.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scopewill be pointed out in the appended claims.

' utilized to explain certain of the operating characteristics oi thearrangement 01 Fig. whileFig.

4 represents a portion of a variable-frequency oscillator embodying amodified form of the invention.

Referring now more particularly to the drawings, the circuit of Fig. 1comprises a variablefrequency oscillatory system, in accordance with theinvention, as utilized in an arrangement for indicating the presence ofa hidden mass of material of such nature as to have a different effectupon an impedance network of the oscillatory system than the substancesurrounding the material. The oscillatory system comprises an impedancenetwork having input terminals M, N, and output terminals 0, P, andproviding in the absence of an external frequency-control condition aminimum value of one component of coupling between the input terminalsM, N and the output terminals 0, P. This network includes a variableimpedance means which is responsive substantially only to the occurrenceof the aforementioned condition for varying the aforementioned onecomponent of coupling. This means comprises a first exploring inductor land a second exploring inductor H with the latter so disposed withreference to inductor I 0 that the effective coupling therebetween isnormally inappreciable but is increased to an appreciable value in thepresence of the external frequency-control condition. This lattercondition, in the particular arrangement shown, is one arising from thepresence in the vicinity of the inductors III, II of the aforementionedhidden mass to be detected. Speciflcally, the inductors l0 and I l aredisposed with their axes normal to each other and so that the axis ofinductor ll passes through the center of inductor 10. This dispositionof two inductors is a well-known arrangement in which there is nocoupling between the inductors.

The variable-frequency oscillatory system of Fig. 1 also comprises anoscillator for producing oscillations and means for applying thegenerated oscillations to the input terminals M, N. As illustrated inthe drawings, this oscillator is of the Hartley type and the oscillatortube thereof comprises the triode section of a triode pentode l2. Theoscillator comprises a frequency-determining circuit including acapacitance l3 and an inductor I, to the midtap of which is connectedthe anode supply voltage for tube l2 through a resistor l5 by-passed toground by a condenser l5. One end of inductor I4 is connected to theanode of the triode section of tube l2 while the other end of inductor His coupled to the grid of this tube section through a condenser H.

In order to excite exploring inductor II with the oscillations producedby the oscillator just described, an inductor I8 is inductively coupledto inductor l4 and is connected through the terminals M, N and throughleads I! to the terminals of exploring inductor l0. These leads areshielded in a suitable manner, as indicated by elements 20 which maycomprise grounded metal tubing, and an inductor 2| is series connectedtherein between inductor I8 and exploring inductor II for a purposewhich will be described more fully hereinafter.

The variable-frequency oscillatory system of Fig. 1 also comprises meansfor amplifying the oscillations, translated through the impedancenetwork having input terminals M, N and output terminals 0, P, and meansfor applying the amplifled oscillations back to the oscillator,comprising the triode section of tube I2, in such phase as to shift theoscillator frequency in response to variations of a component ofcoupling between terminals M, N and O, P.

Oscillations may, under certain operating conditions of the systempresently to be considered, be developed in inductor I I due to theoscillations applied to the inductor l0, and in order that theoscillations in inductor l'l may be used to vary the frequency of theoscillator as described, these oscillations are applied through anamplifier including tubes 23 and 23a, connected in cascade, to the inputcircuit of the pentode section of tube l2, which pentode sectioncomprises a control tube for varying the effective reactance introducedin the frequency-determining circuit l3, l4.

Tubes 23 and 23a are of the duplex-triode type and the input circuit ofthe first triode of tube 23 is coupled through a transformer 25, 25 tothe terminals of inductor H. The midtap of the primary winding 25 isgrounded and the leads from the winding 25 to the inductor II areshielded, as indicated by the element 20, in the manner described above.An inductor 28 is serially included in this circuit for a. purpose to bedescribed more fully hereinafter.

The output circuit of the first triode section of tube 23 iscoupled bymeans of a load resistor 28 and a coupling condenser 30 to the inputcircuit of the second triode section of tube 23, a grid leak 3| beingprovided for the input electrode of the second section of the duplextube 23.

The signal output of the second triode section of tube 23 is, in turn,coupled, by means of a load resistor 32 and a coupling condenser 33, tothe input circuit of the first triode section of tube 23a, a suitablegrid-leak resistor 8 being provided for this first section of tube 23a.

The output circuit of the first triode section of tube 23a is coupled,by means of a load resistor 29a and a' coupling condenser 30a to theinput circuit of the second triode section of tube 23a, a suitablegrid-leak resistor 22 being provided for the control electrode of thelatter section. The output circuit of the second triode section of tube23a is coupled to the input circuit of the pentode section of tube [2 bymeans of a load resistor 32, a coupling condenser 33a, and a grid-leakresistor 3.

The two anodes of tube I 2 are directly connected and a -degree phaseshift is provided for the feed-back control circuit of the oscillator bytuning the secondary winding of the transformer 25, 25 by a condenser 1to the nominal frequency of the oscillations generated by the triodesection of tube l2; that is, the net phase shift in the feed-backcircuit of the oscillator which comprises inductors i0 and II is an oddmultiple of 90 degrees.

The indicator arrangement, per se, of Fig. 1 includes means responsiveto the variation-s in the frequency of the oscillations produced by theoscillatory system above described for indicating the amount ofvariation of the above-mentioned component of coupling caused byvariations in the impedance means including inductors Ill and H.Specifically, the amount of variation of the reactance component ofcoupling isused to indicate the presence of a mass of material of suchnature as to have a different effect on the impedance network, includingvariable impedance means III, II, than the substance of itssurroundings. This last-named means comprises a second oscillator andmeans responsive to variations of a difierence frequency derived fromthe second jack.

- either an audible or a visible indication. This second oscillator isalso 01 the Hartley type and includes a frequency-determining circuitIi, 88 and the triode section or a diode-triode-pentode tube 21. It isdesirable'that a large difi'erence frequency be obtained from arelatively small change of frequency of the first'oscillator. This isefiected by tun-ing the frequency-determining circuit 35, 38 oi! thesecond oscillator to the seventh harmonic of the nominal frequency ofthe first oscillator. One end of inductor 35 is coupled to the controlgrid of the triode section of tube 21 through a condenser 28 and theother end oi. inductor I! is connected directly to the anode of thissection of tube 31. Unidirectional operating'potential for the triodesection of tube 31 is supplied through a resistor 39 connected to themidtap of inductor 25, a suitable by-pass condenser "being alsoprovided.

In-order to combine the oscillations developed in tube l2 with theoscillations developed in tube 81. the-seventh harmonic of theoscillations developed in tube i2 and the oscillations developed in thesecond oscillator are applied to an amplifier which includes the pentodesection of tube 21. A double-tuned transformer 4|, provided for thispurpose, haswthe primary winding thereof coupled through condenser 42 tothe frequencydetermining circuit i3, i4 and the secondary windingthereof connected directly to the control gr-id oi the pentodesection oftube 31. the oscillations developed by the second oscillator being alsocoupled by means 0! a'condenser 43 to the control grid of the pentodesection of tube 31. The condenser is shown in dotted lines for thereason that itmay bemade up in whole or in part of the interelectrodecapacitance of tube 31, Trans former ll is tuned to the seventhharmonic-frequency component developed by the first oscillator, Thesignal output from the pentode section of tube I1 is coupled through thetransformer ILwhich is also tuned to the seventh harmonic frequencydeveloped by the first oscillator and thus'to the frequency 01' thesecondo'scillator, to the diode rectifier including the anode 46 withinthe envelope or the tube 31 wherein the os-- cillations derived from thefirst and second oscillators are heterodyned. The loadcircuit for therectifier includes resistors l'l and 48 connected in series and havingassociated therewith by-pass condensers 49 and 50, respectively,

The rectified beat note or the heterodyned ospath of motion, it issubstantially uncoupled with 9 5 r ode oi thepentode section of tube";Resistor 62 is by-passcd by a condenser "and resistor 83 is by-passed bya condenser I, The voltage developed across resistor II is rectifiediinthe .i diode rectifier. including mode 61 within the envelope oitube 52,and the rectified voltage is applied to the input circuit of theduplex-triode amplifier", resistor 8| thus comprising the load circuitvfor the diode. which includes anode l'l.

The triode sections of tube [I are efiectively con nected in paralleland the voltage developed across resistor ll is coupled to the controlelectrodes oi. the tube through a resistor I and a filter condenser 14,while the meter I is included in the common output circuit or the twotriode sections.

Suitable operating potentials'are provided tor the tube circuits justdescribed by means of a source of B supply I0 anda source of filament orA upply 1|, the filament circuits oi the tubes being completed through athree-position switch "comprising contact points 41,11 and c. Thefilament or tube 23 is connected directly to conto contact b, thefilaments of oscillator tubes l2 and .31 being connected to contact bthrough a choke II. The circuit is thus such that, in one position ofthe movable member or switch 12. the circuit is fully operative and inanother p0 sition thecontact c is disconnected from the source orfilament supply Ii, thereby de-energizing the filaments of tube 22,

While it is intended that the coupling between inductors lliand llnormally be inappreciable, it

has been found that, as a practical matter, this coupling is usuallynormally 0! a very low but appreciable value and that it is necessary toprovide an: auxiliary arrangement for reducing the eil'ective couplingbetween exploring inductors ll and ,l] to an inappreciable value. It isfor this purpose that the inductors 2i and 20 are provided. Theseinductors are physically positioned normally to have substantially nocoupling therebetween. Between these inductors there is dis-' posed amovable inductive coupling means such as a metal disc I! so disposedwith reference to the inductors 2i and 28 that, in a given position inits at least one of inductors 2i and 28, and, in another position in itspath of motion, it isqsubstantially coupled with bothot inductors 2i and28. The

cillations, which is developed across load resistor l8, Ls coupled bymeans-oi a coupling condenser M to the input circuit of the triodesection of a diode-triode-Dehtode tube 52 for amplification. The signaloutput oithe triode section of tube 52 is coupled, through a circuitincluding load resistor 53 and a coupling condenser 54, to the pentodesection of the tube 52 for further amplifi'cation.

The anode of the pentode section of tube 52, which section is connectedas a triode, is connected to the primary winding 55 of an audio-Irequency transformeriii, 56, to the secondary winding 56 of whichheadphones 51 are adapted to be connected by means of a suitable plugand In order to provide a visual indication of the frequency variationsof the first oscillator there is provided a meter to which a current issupplied through the mediumoi' a tube 68 and a suitablefrequency-responsive network, comprising resistors ii, 62, 63, coupledin series through -a condenser 64 between the anode. and thecathmovable'inductive coupling means I! is adapted to be rotated about acenter 16 and a means, comprising an operating arm ll of insulatingmaterial, is rigidly fastened to inductive coupling means II, and ascrew I8 threaded into a conductive shield 19, is provided for adjustingthe coupling between inductors 2| and 28. The adjustable couplingsystem, comprising elements 21, 28, Ii, l6, 11, ll,

may be constructed in accordance with one oi the arrangements describedin applicant's United States Letters Patent entitled Adjustable couplingsystem," No. 2,312,687, granted March 2,

exists between inductors I. and- I: v I further be assumed that theseventhharmonic'frequency of the first oscillator 'is equal to thefrequency of the oscillations'produced by the second oscillator. Underthe conditionsassumed, no oscillations are induced in inductor II frominductor I and hence no voltage is fed-back from inductor II to thefrequency-determining circuit I3, I4 through the circuit comprisingamplifiers "and 23a, and the control or pentode section of tube audiofrequency resistor 81 is eflectlvely bypassed by condenser 86 atrelatively high audio frequencies. This arrangement provides afrequency-responsive network such that a signal I2. However, in case amass of material having an efi'ective permeability different from thatin the general vicinity of exploring inductor I0 is brought intoproximity with inductor In, some voltage is induced in inductor II fromthe inductor III. The

' magnitude o'ithis voltage, and the amplitude of tion of tubelllsvaried in accordance with the amplitude of the oscillations induced ininductor I I. If, on the'o'ther hand, a mass of material having highconductivity-is brought into close proximity to the exploring inductorsIII, II, thecoupling between inductors :IIL and-JI is varied asdescribed and oscillations are induced in inductor II. However, in thiscase if the position of the mass of materiaris not changed theoscillations induced in inductor II are of opposite polarity than is thecase for a mass of actual permeability higher than that of itssurroundings-resulting in a frequency change in the oscillator'sectionof tube I2 in the opposite direction.

Actually, if the mass of materal to be detected is disposedsymmetrically about the axis of the winding of inductor II, no voltageis induced in inductor II from inductor I0. However, in general, ifexploringinductors In and II are carried along in close proximity to theground in a search for a hidden mass of material of the type underconsideration, it is inevitable that, at least during a portion of thetime, the'mass of material to be detected will not be symmetricallydisposed with relation to the axis of inductor I I.

Due to the fact that tubes 23 and 23a are re sistance coupled, there isvery litle over-all phase shift in these amplifiers and the 90-degreephase shift, introduced in the circuit by the tuning of transformer 26by condenser I insures that only a reactance component is introducedinto the frequency-determining circuit I3, I I.

Any variation of the normal frequency of the first oscillator causes abeat note to be produced in the output circuit of the diode section oftube 31, due to the heterodyning of the oscillations developed by tubeI2 and the oscillations developed by the second oscillator. This beatnote is detected in the diode section of tube 31 and a voltage varyingin accordance therewith is developed across resistor 48 in the mannerdescribed above. This audio-frequency voltage is, in turn, amplified inthe two sections of tube 52 in order to produce an audible signal at theheadset 51, thereby to indicate the presence of the mass of material tobe detected.

The audio-frequency voltage developed in the output circuit of thepentode section of tube 52 is also applied to the impedance networkcomprising elements $I66, inclusive. Condensers 65 and 88 are preferablyof such values that resistors 82 and 63 are effectively not by-passed atlow audio frequencies'while resistor 62 is effectively lay-passed bycondelfifiilffi at an intermediate having an amplitude varying inaccordance with frequency is supplied to the diode section of tube 52.Tube 68, in turn, amplifies the unidirectional component of therectifiedsignal developed across the resistor Q I by the diode sectionof tube II to o'ovide a. visual\ frequency indication on meter Variousother positions of inductors III and II provide zerocoupling between theinductors and are, therefore, suitable for use in a variable-frequencyoscillatory system of the type just described. some of these positionsand the manner of determining the same are disclosed in theabove-mentionedv United States Letters Patent. However, in any practicalsystem it is diflicult to provide exactly zero coupling betweenexploring inductors I0 and II and it is for the puri pose iof reducingthe effective coupling therec f 'i 7..v V means; including inductors IIand II remote fromfexplorlng inductors I0 and II, has been proanegligible value that the auxiliary vided. Inductors 2| and 28 are alsopositioned,

- as above described, to have substantially no couplin'g therebetweenand the coupling between inductors 2I and Ml-can be varied by adjustingthe coupling means I5 in a manner which is fully described in theabove-identified United States Letters Patent, rendering furtherdescription thereof unnecessary in this specification. This adjustmentin turn serves to reduce to zero the normal effective coupling betweeninductors I and II.

The adjusting means including condenser 8| has been provided so that aninitial adjustment of the detector system itself can be made and so thatthe latter can be tested and adjusted from time to time. Suchadjustments may be necessary due to aging of the tubes, etc. In order soto adjust the circuit, switch 12 is operated so that contact 0 is open,thus efiectively opening the filament circuit of tube 23. In thiscondition, the portion of the variable-frequency oscillatory systemwhich includes inductors III, II, transformer 25. 26, amplifiers 23,23a, and the control tube including the pentode section of tube I2, iscompletely inoperative and the frequency of the second oscillator isadjusted by means of condenser 8| so that it is exactly equal to theseventh harmonic frequency of the first oscillator. Under theseconditions no audible beat note is produced at headset 51, and noindication is provided by meter 60. After this adjustment has been made,the contact 0 is closed, as shown in the drawings, rendering thevariable-frequency oscillatory system fully operative. An audio signalwill now be heard in headset 51 or indicated b meter-60 if there is 'anyresidual undesired coupling between exploring inductors I0 and II. Theadjusting screw I8 is adjusted until this beat note is no longer presentand the set is ready for operation, as described above.

Fig. 2 comprises a graph utilized to illustrate the frequency shift ofthe controlled or first oscillator and the resulting beat frequencybetween the harmonic of the oscillations developed in tube I2 and thoseof oscillator 31 when a metal object is moved in a plane below inductorII and normal to the axis thereof. Thus, the dotted curve A representsthe frequency change of the first oscillator when the metal object ismoved indication.

from the left to the axis of inductor Ii, while dotted curve Billustrates the frequency change when the object is moved from the axisof inductor II to the right. Corresponding beat frequencies obtained arerepresented by curves A. B, respectively. t

Fig. 3 comprises a graph utilized to illustrate the frequency shift ofthe first oscillator and the beat frequency developed when a metalobject to the right or left of the axis of inductor ii is moved forwardor backward parallel to the axis of inductor Iii. Thus, curve Frepresents ing to the conditions represented by curves F arrangement ofthe Fig. 1 type by disconnecting,

and E are illustrated by curves D and C, respectively.

exploring inductors i0, ii which may be about 12 inches or 15 inches indiameter are carried about six inches above the ground. Under normalconditions over barren ground there is no couplin between the inductorsIII, II and no control voltage is supplied to the pentodesection of tubel2. When a metallic body or a body having a permeability different fromthat of the ground being explored is in the immediate field oftheinductor lli, currents are produced in inductor i0 which produce anauxiliary field. This field is unsymmetrical and induces a small voltagein inductor II. The net efiect is to produce a small effective couplingbetween inductors Ill and II through the body to be located. Thearrangement] is preferably designed so that the resultant beat frequencydeveloped in the diode section of tube 31 approaches the upper limit ofaudibility when the object to be located is close to exploring inductorsl0, II. A metal obiect just below the surface of the ground provides amaximum indication when it is just to the right or left of the positiondirectly below the exploring inductors I0, -I I. When thecenter of theobject is directly below the center of inductor ii or directly ahead orb'ehind'that position, no indication is provided by the arrangement.However, if the exploring inductors are moved to the right or to theleft of a position directly over an object to be located, a largeindication is produced. A very accurate location of the object is,therefore, possible. The actual frequency shift of the first oscillatorand the output indications increase as the size of the object isincreased or when the object is buried in the ground at a shallowerdepth. The arrangement is preferably so proportioned that the amplitudeof the beat-frequency voltage is just below the overload point of thebeat-frequency amplifier tube 52 regardless of the frequency of the beatnote. The ability to detect a small object by from terminals M. N and O,P, the elements II, II, II, II, 2!, 25, 28 and 1 and by connectingcorrespondingly lettered terminals of Fig. 4 to the terminals of Fig. 1.In place of the pair of exploring inductors l0, ll of Fig. 1, thearrangement of Fig. 4 provides a single exploring inductor i0 connectedin one arm of a reactance bridge, another arm of which comprises a fixedinductor ll. Oscillations are supplied to inductor Ill from the firstoscillator through the terminals M, N and means are provided forderiving oscillations from the impedance network having a characteristicwhich varies with the reactive efi'ect of inductor III on the network orinductance bridge. 1 This last-named means comprises a tube 86 having aninput circuit connected across one diagonal of the reactance bridge andhaving an output circuit coupled through transformer 81 to the terminals0, P. An auxiliary acUusting arrangement, including inductors II, 28'and 15' corresponding inall respects to those of similarly designatedelethe art. The system is initially so adjusted that.

in the absence of a mass of material of either of the types describedabove in the vicinity of inductor II, the inductance bridge comprisinginductor II' in one arm and inductor 85 in another arm is effectivelybalanced so that no voltage is sup.-

.plied to the input circuit of tube 86. If necessary,

the coupling means I! is adjusted as described with reference to Fig. 1in order to effect the initial balance of the bridge. The presence of aconductive body or a body having a permeability different from thepermeability of the medium surrounding inductor i0 is effective to upsetthe balance of the bridge, thus causing a voltage to be supplied to theinput circuit of tube 86, which means of the headphones, therefore,depends only on the sensitivity of the huinan ear to low-frequencynotes. The frequency heard in the headphones increases as the object tobe located, is approached, thus giving an easily distinguishable Thevariable-frequency oscillatory system is preferably proportioned so thatoverloading of the tubes 23 and 23a in its feed-back network limits themaximum frequency shift which may be obtained. The meter 80 is providedfor the reason that it is more sensitive at low frequencies than thehuman ear. The oscillatory system of the invention, being primarilyvoltage in turn is utilized to vary the frequency of the firstoscillator in a manner similar to that described above with reference tothe description of the operation 02; the circuit of Fig. 1.

Whfle in each of the arrangements described, the effect of the feedbackcircuit on the first oscillator is to vary the frequency of the firstoscillator from some predetermined given frequency, it will beunderstood that an embodiment of the invention may comprise anarrangement in which the first oscillator is normally inoperative but isconditioned for operation and caused to oscillate due to the action ofthe feed-back circuit, its frequency being thereafter controlled asdescribed above. It will be understood that the terminology "shift thefrequency of the oscilll later" is intended to apply also to anarrangement of this type.

While applicant does not intend to be limited to any particular circuitvalues in the embodiments of the invention described, there follows aset of circuit values which have been found to be particularly suitablefor the variable-frequency oscillatory system and the detectorarrangement of Fig. 1:

Inductors Ill and l I are wound in grooves in rigid wooden coil formsand rigidly mounted at right angles to each other, as described.

Inductor l I 3 Load resistors 29, 32, 29a and 32a kilohms- 150 Couplingcondensers 30, 33, 30a and 33a micromicrofarads 20 Grid-leak resistors3|, 8, 9 and 22 kilohms 100 Resistor IL... kilohms.. 2.2 Condenser l6microfarads 0.01 Condenser l1 micromicrofarads 200 Condenser 42 -do..-20 Condenser 38 do 100 Condenser I4 ..microfa.rads 0.02 Condenser 80micromicrofarads 50 Condenser 8| ..do.. 15 Resistors 41 and 48 kilohms100 Condensers 49 and 50 micromicrofarads 100 Resistors GI, 62 and 63kilohms 220 Condenser G mlcrofarads 0.0001 Condenser 66 do- 0.001Condenser 64 do 0.02 Resistor 69 megohms 1 B supply source I0 volts 90 Asupply source Ii ..do 1.5 Frequency of first oscillator kilocycles 100Frequency of second oscillator do 700 While the variable-frequencyoscillatory system has been described as utilized in an arrangement forlocating a hidden mass of material, it will be understood that theinvention is not intended to be limited to such application. Theoscillatory system may. for example, be arranged to respond tovariations in an impedance network due to variations in temperature of avariable impedanc means included in the network the invention.

What is claimed is:

1. A variable-frequency oscillatory system comprising, an impedancenetwork having input and output terminals and including a passivemutualreactance network normally balanced to provide a substantiallyzero value of feed-back voltage from cy-control condition externalthereto in space for modifying said balance of said network to providefeed-back voltage from said input to said output terminals. anoscillator for generating oscillations, means for applying saidoscillations to said input te means for amplifying the oscillationstranslated through said network to said output terminals, and means forapplying the amplified oscillations back to said oscillator in suchphaseas to shift the oscillator frequency in response to said modification ofsaid mutual-reactance network balance.

- 2. A variable-frequency oscillatory system comprising, a reactancenetwork having input and'output terminals and/including a passivemutual-reactance network normally balanced to provide a substantiallyzero value of a reactive component of coupling between said input andsaid output terminals, said mutual-reactance network being responsive toa frequency-control condition external thereto in space for modifyingsaid' baalnce of said network to provide a reactive component ofcoupling between said input and said output terminals, an oscillator forgenerating oscillations, means for applying said oscillations to saidinput terminals, means for amplifying the oscillations translatedthrough said network to said output terminals, and means for applyingthe amplified oscillations back to said oscillator in such phase as toshift the oscillator frequency in response to variation of said reactivecomponent of coupling.

3. A variable-frequency oscillatory system having an operating frequencyvaried in response to the presence of a hidden mass of material of suchnature as to have a different effect on an impedance network than doesthe substance surrounding the material comprising, an impedance networkhaving input and output terminals and providing in the presence of saidsubstance but in the absence of said mass of material a minimum value ofone component of coupling between said input and output terminals, anexploring inductor included in said network and responsive substantiallyonly to said mass of material for varying said one component ofcoupling, an oscillator for generating oscillations, means for applyingsaid oscillations to said input terminals to excite said inductor withsaid oscillations, means for amplifying the oscillations translatedthrough said network to said output terminals, and means for applyingthe amplified oscillations back to said oscillator in such phase a toshift the oscillator frequency in response to variation of saidcomponent of coupling.

4. A variable-frequency oscillatory system having an operating frequencyvaried in response to the presence of a hidden mass of material of suchnature as to have a different effect on an impedance network than doesthe substance surrounding the material comprising, an impedance networkhaving input and output terminals and providing in the presence of saidsubstance but in the absence of said mass of material a minimum value ofone component of coupling 1 between said input and output terminals, 9.pair said input to said output terminals, said mutualof exploringinductors included in said network and so disposed that the effectivecoupling between said inductors is normally low but is varied to anappreciable value in the presence of said mass of material to vary saidone component of coupling, an oscillator for generating oscillations,means for applying said oscillations to said input terminals, means foramplifying the oscillations translated through said network to saidoutput terminals, and means for applying the amplified oscillations backto said oscillator in such phase as to shift the oscillator frequency inresponse to variation of said component of coupling.

5. A variable-frequency oscillatory system having an operating frequencyvaried in response to the presence of a hidden mass of material of suchnature as to have a different effect on an impedance network than doesthe substance surrounding the material comprising, an impedance networkhaving input and output terminals and providing in the presence of saidsubstanc but in the absence of said mass of material a minimum value ofone component of coupling between said input and output terminals, apair of exploring inductors included in said network and so disposedthat the axis of one of said inductors is normal to the axis of theother, thereby to render th eiiective coupling between said inductorsnormally low but variable to an appreciable value in the presence ofsaid mass of material to vary said one component of coupling,

an oscillator for generating oscillations, means for applying saidoscillations to said input terminals, means for amplifying theoscillations translated through said network to said output terminals,and means for applying the amplified oscillations back to saidoscillator in such phase as to shift the oscillator frequency inresponse to variation of said component of coupling.

6. A variable-frequency oscillatory system having an operating frequencyvaried in response to the presence of a hidden mass of material of suchnature as to have a difierent effect on an impedance network than doesthe substance surrounding the material comprising, an impedance networkhaving input and output terminals and providing in the presence of saidsubstance but in the absence of said mass of material a minimum value ofone component of coupling between said input and output terminals, apair of exploring inductors included in said network and so disposedthat the axis of one inductor is normal to the axis of the otherinductor and in line with the center of said other inductor, thereby torender the effective coupling between said inductors normally low butvariable to an appreciable value in the presence of said mass ofmaterial to vary said one component of coupling, an

oscillator for generating oscillations, means for applying saidoscillations to said input terminals, means for amplifying theoscillations translated through said network to said output terminals,and means for applying the amplified oscillations back to saidoscillator in such phase as to shift the oscillator frequency inresponse to -variation of said component of coupling.

7. A variable-frequency oscillatory system having an operating frequencyvaried in response to the presence of a hidden mass of material of suchnature as to have a different effect on an impedance network than doesthe substance surrounding the material comprising, an impedance networkhaving input and output terminals and providing in the presence of saidsubstance but in the absence of said mass of material a minimum value ofa reactive component of coupling between said input and outputterminals, a pair of exploring inductors included in said network and sodisposed that the reactive coupling between said inductors is normallylow but is varied to an appreciable value in the presence of said massof material to vary said reactive component of coupling, means foradjusting said normally low coupling so that it is inappreciable, anoscillator for generating oscillations, means for applying saidoscillations to said input terminals, means for amplifying thoscillations translated through said network to said output terminals,and means for applying the amplified oscillations back to saidoscillator in such phase as to shift the oscillator frequency inresponse to variation of said component of coupling.

8. A variable-frequency oscillatory system comprising an impedancenetwork having input and output terminals and including a passivemutual-reactance network normally balanced to provide a substantiallyzero value of feed-back voltage from said input to said outputterminals, said mutual-reactance network being responsive to afrequency-control condition external thereto in space for modifying saidbalance of said network to provide feed-backvoltage from said input tosaid output terminals, an oscillator for generating oscillations, meansfor applying said oscillations to said input terminals, means foramplifying the oscillations translated through said network to saidoutput terminals, means for applying the amplified oscillations back tosaid oscillator in such phase as to shift the oscillator frequency inresponse to said modification of said mutual-reactance network balance,and an auxiliary means remote from said impedance means for adjustingthe effect of said impedance means on said network.

9. A variable-frequency oscillatory system comprising, an impedancenetwork having input and output terminals and providing in the absenceof an external frequency-control condition a minimumvalue of onecomponent of couplin between said input and output terminals, im-Dedance means included in said network and responsive substantially onlyto the occurrence of said condition for varying said one component ofcoupling, an oscillator for generating oscillations, means for applyingsaid oscillations to said input terminals, means for amplifying theoscillations translated through said network to said output terminals,means for applying the amp ified oscillations back to said oscillator insuch phase as to shift the oscillator frequency in response to variationof said component of coupling, and means for adjusting the effect ofsaid impedance on said network comprising a first inductor, a secondinductor s0 disposed in the ma netic field of said first inductor thatsaid second inductor is substantially uncoupled with said firstinductor, a movable inductive coupling means so disposed with referenceto said first and second inductors that in agiven position in its pathof motion it is substantially uncoupled with at least one of said firstand second inductors and in another position in its path of motion it issubstantially coupled with both of said first and second inductors, andmeans for moving said coupling means in said path to adjust the cou-Pling between said first and second inductors.

LESLIE F. CURTIS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENT

